Hi Ashwini,
I understand that the cost of the trip needs to be thought about, however, I will still post travel information by train and flight, as then you have a choice of travel, and can easily compare, the time it takes to travel and also cost. Below are travel details from Mumbai to Sasan Gir (Gir National Park).
Mumbai to Gir National Park by Flight
Going by flight from Mumbai to Gir National Park would be the easiest and quickest way. As there is no airport in Sasan Gir there are two destination options for flights.
Flight from Mumbai to Rajkot or to Diu.
Mumbai to Rajkot
Mumbai to Rajkot flight would take around 1 hour and 10 minutes. From Rajkot, you would then need to take a taxi towards Gir National Park which would take roughly 4 hours.
Mumbai to Diu
Mumbai to Diu flight would take roughly the same time around 1 hour 5 minutes. However the travel time from Diu to Gir National Park is less, and it would only take you around 2 hours to reach the park from the airport.
Mumbai to Gir National Park by Train
Mumbai to Junagadh
If traveling by train, from Mumbai you would need to get a train to either Junagadh Junction. The journey would take anywhere between 17 hours to 1 day and 16 hours, depending on which train you choose. From the Junagadh Junction railway station, you would then need to get a taxi to Gir National Park which would take another 2 hours.
Mumbai to Rajkot
The other option is getting a train from Mumbai to Rajkot (mainline railway station). This train journey would take you anywhere between 11 and a half hours to 14 hours, again depending on which train you take. But when wanting to reach Gir National Park from Rajkot railway station the journey would take you 4 hours by taxi.